Sending from multiple domains

I've just installed ZCS FOSS edition, and I have to say I'm very impressed - the whole application is extremely slick-looking, about to try Zimbra Desktop, but that's another story...

I'm thinking about deploying the FOSS version to one of my clients who is on an extremely tight budget, but they have a requirement that a handful of users need to be able to send (and receive) from more than one mail domain, as they use multiple brands when dealing with their customer base. I have set up aliases to handle the incoming mail, but can someone tell me how I would go about sending mail from one of these aliases? Is it even possible?